I wear some cheap tennis socks. As long as they’re clean to start with, they seem fine, were 3 pairs for a fiver from jd sports or somewhere like that.
If it is really cold I have some cold weather cycling socks (wooly boolies I think), and if it is snowy I’ve got hiking socks which do the job.
I’m a bit sceptical about the whole gait analysis / corrective shoes business – I’ve bought shoes from the same shop twice and been analysed, and been told that I need pronation correcting shoes once, and neutral shoes the other time. I just buy neutral or less cushioned shoes. Fit is important in the same way as any other shoe – you don’t want blisters when you get wet, but if the whole corrective shoes thing was true people wouldn’t get so many injuries any more when they clearly do.
Same for the whole chuck out shoes after a couple of hundred miles thing, always seems like something made up by running shops and shoe manufacturers – I change mine when they have too many holes, and they seem fine to run in (and I don’t get injuries).